1. Field of the Invention
The present invention relates to a holographic projection method and a holographic projection device.
2. Description of the Related Art
In general, examples of a projection device that projects a video image onto a screen include a projection device using transmission type liquid crystal (LC), a projection device using reflecting liquid crystal (liquid crystal on silicon [LCOS]), and a projection device using a digital micromirror device (DMD).
For example, in the projection device using liquid crystal, a video image is projected onto a screen as follows. That is, first, a video image that a user desires to project is displayed on liquid crystal in accordance with video image data. Then, the liquid crystal is illuminated by illumination light and transmission light or reflection light is projected by a projection lens, and the video image displayed on the liquid crystal is enlarged and displayed on a screen to be enlarged.
In the projection device using a DMD, a video image in displayed on the DMD by turning on/off the micromirrors which constitute the DMD and correspond to pixels in accordance with video image data and then the video image is projected onto a screen by a projection lens.
The above-described projection devices adopt a method in which a video image is displayed on an image display element, e.g; using liquid crystal at a time and the displayed video image is projected to be enlarged, and by the projection lens. Accordingly, the above-described projection devices each include a light source, an illumination optical system, an image display element, and a projection lens.
In addition, in a color sequencing expression, a color filter that changes a color of a light source is further required. In a method that uses a plurality of image display elements for each color, a color separation/synthesis optical system that is provided around the image display elements is further required.
In general, an incoherent light source, such as a high pressure mercury lamp, is used as a light source. Therefore, as the light source, a complicated illumination optical system that efficiently and uniformly illuminates light to the image display elements is needed. As a result, the size of the illumination optical system is increased, which results in increasing manufacturing costs.
In addition, a projection lens having a high precision is needed to project high-definition video images that are displayed on the image display elements. As a result, the size of the projection lens is increased, which results in increasing manufacturing costs.
For a color conversion, a color filter needs to be provided or a color synthesis/separation optical system needs to be provided around the excessively complicated image display elements. Accordingly, the size of the projection device and the manufacturing costs may be increased.
In view of such circumstances, a projection device (called a holographic projection device) using a spatial light phase modulator (SPM), which is shown in FIG. 7, is suggested. For example, this type of projection device is disclosed in detail in WO 2005/059881A3.
That is, in the projection device that is disclosed in WO 2005/059881A3, as shown in FIG. 7, linearly polarized light from a light source (laser) 100 is incident on a polarized beam splitter (PBS) 102, reflected on the PBS 102, and incident on an LCOS 104 that is the SPM. In addition, a λ/4 plate (not shown) is provided between the PBS 102 and the LCOS 104.
Diffraction light 105 is subjected to phase modulation by the LCOS 104 in accordance with video image data and then reflected. The diffraction light 105 passes through the λ/4 plate again, is transmitted through the PBS 102, and is projected onto a screen 108 through a projection lens 106. In this case, a binary modulation of a phase difference π is obtained according to whether or not the phase modulation is performed by the LCOS 104. Meanwhile, zero-order light 107 is not incident on the projection lens 106.
As such, for example, in WO 2005/059881A3, a small-sized projection device is disclosed, which uses a simple illumination optical system and includes a simple projection lens.
The method that is disclosed in WO 2005/059881A3 is a method that projects a video image using diffraction. In this case, the brightness of the projected video image is determined based on diffraction efficiency of a spatial light phase modulator. For example, in the case of the binary modulation of the phase difference π, diffraction efficiency is approximately 40%. In addition, if the amount of phase modulation is changed minutely, it is possible to improve diffraction efficiency. In addition, if the amount of phase modulation is continuously changed, the diffraction efficiency ideally reaches 100%.
In addition, the total amount of the diffracted light with respect to the light source occupies a predetermined ratio. However, a bright scene and a dark scene exist in the video image, and the total amount of light is changed. Accordingly, when the brightness of the light source is constant, for example, an originally dark scene of a video image may be displayed bright. For this reason, the total amount of brightness of each scene needs to be calculated for each scene based on video image data, and the amount of light that is incident on the SPM needs to be adjusted such that the brightness of each scene is appropriately maintained with respect to a scene having the maximum brightness.
As a technology that may be used to solve the above-described problems, for example, U.S. Pat. No. 5,589,955A discloses the following technology. In the case that characters of dot patterns are drawn, since the number of dots is different for each character, an output of a laser that is a light source is increased or decreased make brightness of each character the same by counting the number of the dots.